Typical brushless DC motors are controlled by electronics due to the lack of the self commutation brushes. Electronic systems driving brushless DC motors use the knowledge of the rotor position in order to commutate properly. Hall-effect sensors, one of the popular choices, are used to decode the position of the rotor as it advances. The disadvantages of the Hall-effect sensors are cost, mounting spaces, and performance degradation due to aging. A sensorless motor start up would overcome the problems presented above. A sensorless motor startup would acquire the position information as the rotor advances without the use of external sensors.
One of the sensorless startup methods is based on the principal that the inductance of the rotor is a function of its position. In particular, the relationship of the inductance and the rotor position depends on the stator, the rotor, the direction of the current, and the mechanical structure of the motor itself. Generally, the inductance, measured from phase-to-phase, is a function of the rotor's position, which includes the primary inductance and the mutual inductance of the motor. The primary inductance usually depends on the length of the wire, material properties of the wire (i.e., conductivity), and properties of the coil (i.e., number of turns), and the mutual inductance usually depends on the change of the magnetic field the nearby coils. The direction of the current also affects the magnetic field; thus it also affects the mutual inductance as well. As an example, in FIG. 1, an diagram depicting the phase-to-phase inductances for a three-phase brushless DC motor (i.e., LAB for A-to-B inductance, LBC for B-to-C inductance, and LCA for C-to-A inductance) as a function of mechanical angle.
For a three-phase brushless, sensorless DC motor, there are three phases (i.e., phases A, b, and C) that can be driven in six different ways, known as commutation states. The ability to measure the inductance of the six commutation states provides the information to derive the most optimal state to spin up a motor, and, because the inductance has an inverse relationship with the current driven into the coil, the current could be used to infer to the inductance. Namely, the rise time of the current can be used to determine inductance. In FIG. 2, a diagram depicting three inductances L1, L2, and L3 (i.e., 0.5 mH, 0.6 mH, and 0.7 mH, respectively) having current rise times of T1, T2, and T3 (i.e., 0.18 ms, 0.19 ms, and 0.21 ms, respectively).
For startup in a conventional system, complementary commutation states are generally employed as shown in FIG. 3. Inductive rise times for a driving state and its complementary state are compared to one another such that when the inductive rise times cross each other a switching point has been reached. In the example of FIG. 3, the inductive rise times for states 3 and 0 are used to determine the next commutation switching point. These states (states 3 and 0) have use the same terminals, but the difference is the direction of the current flowing through the phases of the motor. As shown in FIG. 4, these measurements occur during an off-time interval TOFF (interval TOFF includes two coast intervals TCOAST and two sensing intervals TSENSE so as to sense the two inductive rise times). This methodology, however, significantly affects the efficiency of the driving torque and power consumption. Thus, there is a need for an improve start-up method and associated apparatus.
Some examples of conventions systems are: U.S. Pat. No. 5,841,252; 7,023,155; 7,589,484; U.S. Patent Pre-Grant Publ. No. 2004/0036436; European Patent No. EP0462729; and U.S. patent application Ser. No. 13/009,538.